Various methods for upgrading hydrocarbons to aromatics with crystalline aluminosilicate zeolite catalysts are generally known. For example, methods for producing gasoline boiling range hydrocarbons from methanol or other lower aliphatic oxygenates (an "MTG" process) are exemplified in U.S. Pat. Nos. 3,998,899, 3,931,349 and 4,044,061. Also, methods for producing gasoline boiling range hydrocarbons from light hydrocarbon feeds containing, e.g., paraffins, olefins and naphthenes have also been developed. Such processes have been labeled as "M2-forming." See, e.g., Chen, et al, "M2-forming--A Process for Aromatization of Light Hydrocarbons", Ind. Eng. Chem. Process Des. Dev., 25, 151-155 (1986). U.S. Pat. Nos. 3,760,024, 4,120,910, 3,845,150 and 4,350,835 are generally illustrative of this type of aromatization process.
The M2-forming aromatization process uses a medium pore zeolite catalyst to upgrade the paraffinic, olefinic, naphthenic or lower aliphatic oxygenate feed to gasoline boiling range hydrocarbons. These types of medium pore size zeolite catalysts will be described in greater detail hereinafter. Further, suitable aluminosilicate medium pore size zeolite catalysts for use in an M2-forming process often have zinc (a dehydrogenation metal) incorporated therein/thereon by means of ion exchange or impregnation techniques.
In an M2-forming process, zinc-impregnated aluminosilicate zeolite catalysts, such as Zn/ZSM-5, are known to be active as well as selective for the M2- forming aromatization reactions. Light hydrocarbons, such as paraffins, olefins and naphthenes are converted to aromatics via complex, consecutive, acid-catalyzed reactions, including (1) conversion of olefinic and paraffinic molecules to small olefins via acidic cracking and hydrogen-transfer reactions, (2) formation of C.sub.2 -C.sub.10 olefins via transmutation, oligomerization, cracking and isomerization reactions, and (3) aromatic formation via cyclization and hydrogen transfer, as explained by Chen et al in the above-noted article on M2forming.
In any event, the desired result of this type of process is the production of gasoline boiling range materials. Gasoline, as such term is used in the instant specification, and as such ter is commonly used in the petroleum industry, is useful as a motor fuel for internal combustion engines. More specifically, gasoline is hydrocarbon in nature, and contains various aliphatic and aromatic hydrocarbons having a full boiling range of about 280.degree. to 430.degree. F., depending upon the exact blend used, and the time of year. Although gasoline is predominantly hydrocarbon in nature, various additives which are not necessarily exclusively hydrocarbon are often included. Additives of this type are usually present in very small proportions, e.g., less than 1% by volume of the total gasoline. It is also not uncommon for various gasolines to be formulated with non-hydrocarbon components, particularly alcohols and/or ethers as significant, although not major, constituents thereof. Such alcohols, ethers and the like have burning qualities in internal combustion engines which are similar to those of hydrocarbons in the gasoline boiling range. For purposes of this specification, the term "gasoline" is used to mean a mixture of hydrocarbons boiling in the aforementioned gasoline boiling range and is not intended to mean the above-referred to additives and/or non-hydrocarbon constituents.
In an M2-forming process, zinc-impregnated aluminosilicate intermediate pore size zeolite catalysts, such as Zn/HZSM-5, are known to be quite effective in attaining high aromatic yields. However, this type of zinc catalyst suffers irreversible loss of activity and selectivity due to zinc volatilization at the high temperatures encountered during conversion. It is known that zinc metal has a high vapor pressure at elevated temperatures. Consequently, zinc catalysts operating in reducing atmospheres at temperatures greater than about 300.degree. C. will lose zinc by volatilization.
Several attempts have been made to prevent zinc elution from such catalysts. For example, U.S. Pat. No. 4,097,367 teaches that adding a second metal (in addition to zinc) selected from Group IB or VIII, such as palladium, to medium pore size zeolite catalysts will inhibit zinc elution from the catalyst in a reducing atmosphere under conversion conditions of temperatures in the range of about 900.degree. to about 1200.degree. F. However, the powerful catalytic metal palladium does not enhance activity of the catalyst in the sense of achieving increased conversion. Similarly. U.S. Pat. No. 4,490,569 teaches that elution of zinc from zinc-impregnated zeolite catalysts can be minimized by incorporating gallium into the catalyst composition, where palladium is optionally also included in the catalyst composition. U.S. Pat. No. 4,128,504 also is directed to a method of preventing the elution of zinc from a zinc-containing zeolite by incorporating therein another metal selected from the group consisting of metals of Group IB and VIII of the periodic table, germanium, rhenium, and the rare earths, e.g., cerium. Examples of such metals from Groups IB and VIII include copper, gold, silver, platinum and nickel. These prior attempts to stabilize zinc involved the formation of an alloy comprising zinc and a second metal, such as Pd or Ga to reduce the zinc vapor pressure.
However, none of these prior attempts have proven completely successful at preventing zinc elution from zinc-impregnated zeolite catalysts. Moreover, the noble metals, etc., employed as elution preventing metals on the catalyst in these above-described methods are expensive, thus making their use less desirable from an economic perspective.
Thus, an effective and relatively inexpensive method of stabilizing the zinc metal on zeolite catalysts so as to result in economically desirable longer catalytic life is still desired.